The present invention relates to temperature measurement devices. In particular, the invention relates to infrared sensors.
Infrared sensors, such as IR spot sensors (or infrared thermometers), are used to provide an output that represents a temperature of a point of interest. Many infrared spot sensors make use of both a thermopile and a resistance temperature detector (RTD). The thermopile produces an output voltage representing the temperature of a point of interest. The RTD, which is used for ambient temperature compensation, requires a current flowing through the RTD to produce a voltage that is a function of the resistance, and therefore a function of the ambient temperature.
To minimize overall lead wire count, the thermopile and RTD of the infrared sensor are each connected by two wires for two independent measurements. When measuring resistive devices, such as an RTD, with only two lead wires, the lead wire resistance generates error in the measurement by directly adding to the overall RTD sensor resistance.
One approach for addressing the error induced by lead wire resistance in a two wire RTD measurement is to use a large (higher resistance) RTD. For example, one infrared thermometer sensing head offered by Optris (model LT15) contains a thermopile and a PT1000 RTD. The PT1000 RTD has a nominal sensitivity of about 30 ohms/DegC, which helps to minimize the effects of lead wire impedance compared to a similar two wire circuit with a smaller RTD such as a PT100 RTD. However, due to its higher resistance, the large RTD sensor will amplify any electromagnetic interference (EMI) currents that it is exposed to, and will also require less resolution for digitization. This ultimately results in a noisier, less accurate two wire measurement produced by a PT1000 RTD ambient compensation resistor as compared to a smaller RTD device, such as a PT100 RTD.